1. Field of the Invention
This invention relates to a zoom lens of two-unit construction comprising, in succession from the object side, a lens unit of negative refractive power and a lens unit of positive refractive power.
2. Related Background Art
In interchangeable lenses for 35 mm still cameras, there is the impression that in recent years, lenses which are zoom lenses of two-unit construction having negative and positive refractive powers in succession from the object side and which include the standard angle of view and have a magnification change ratio of about 1.8 times have completely taken root as lenses standard-mounted in place of standard lenses (in case of 35 mm format, lenses of about 50 mm). Accordingly, a zoom lens of this type is carried while remaining a mounted as a commonly used lens on a camera body and therefore, its minimum downsizing is a requisite condition and further, it is necessary that such lens keep a sufficient imaging performance and yet be compact and inexpensive.
To realize such a so-called standard zoom lens, the zoom lens of two-unit construction as described above is the most suitable lens type, and various lenses of such type have heretofore been proposed. Among the heretofore proposed lenses, there are ones disclosed in U.S. Pat. No. 4,195,912 and U.S. Pat. No. 4,560,253 as examples in which a first lens unit comprises only two lenses, i.e., a negative lens and a positive lens and which has achieved a low cost.
At the present time, however, standard zoom lenses including the standard angle of view and having a magnification change ratio of 2.1 times or greater are becoming the main stream as lenses standard-mounted on camera bodies, and in such lenses, as in the afore-described standard zoom lenses having a magnification change ratio of about 1.8 times, downsizing and low cost are requisite conditions. In the conventional zoom lenses having a magnification change ratio of about 1.8 times, to realize low cost and downsizing while keeping a sufficient performance, zoom lenses of two-unit construction have been suitable. However, when a magnification change ratio of two times or greater is to be secured, it has been difficult to use the conventional compact two-unit lens type as it is, and it has been usual that the lens becomes bulky in order to keep the imaging performance thereof sufficiently. So, the present situation is that efforts are made to make downsizing and a high performance compatible by the use of a zoom lens type of three-unit construction which comprises three negative, positive and negative units and which is of the type in which a third unit is fixed and which is relatively simple in structure.
As described above, in standard zoom lenses having a magnification change ratio of 2.1 times or greater, zoom lenses of three-unit construction have become the mainstream from the viewpoint of downsizing. However, the increase of one unit basically in the unit construction leads to an increase in cost and is not preferable. Also, basically in these three-unit zoom lenses, the type in which a negative unit is installed as a fixed unit rearwardly of a two-unit zoom lens and the back focal length is made small to thereby achieve downsizing is the mainstream, and to make the lens sufficiently compact, it is necessary to make the refractive power of the last negative third unit sufficiently great. Accordingly, it becomes necessary to achromatize in the third unit, and to achromatize sufficiently, it is necessary to increase at least two lenses as compared with a two-unit zoom lens, and apparently this leads to an increased cost. Also, if a zoom lens is forced to be constructed of the same number of lenses as in a two-unit zoom lens, a deteriorated performance will be unavoidable. Also, at the telephoto end, the telephoto ratio (the ratio of the distance from the foremost surface to the focal plane of the lens to the focal length) will become small due to the third unit and therefore a problem in manufacture such as eccentricity will arise, and this is not preferable.
Accordingly, it has been necessary that a zoom lens which is very compact and low in cost and has a good performance and a magnification change ratio of 2.1 times or greater be realized by a two-unit zoom lens.
Also, in the zoom lens disclosed in U.S. Pat. No. 4,195,912, in spite of the magnification change ratio being as small as the order of 1.6-1.8 times, the full length is great and the diameter of the fore lens is also large. Particularly at the wide angle end, the full length becomes remarkably great and therefore, an oblique ray incident on the fore lens passes a location farther from the optical axis and thus, the diameter of the fore lens becomes remarkably large, and this has led to the disadvantage that the zoom lens becomes bulky as a whole. Also, the refractive power of the second lens unit is weak and therefore, the full length becomes much greater, and the amount of movement of each lens unit has been great. In terms also of aberrations, the fluctuation of coma and curvature of image field have not been good for the large size of the entire lens.
In the zoom lens disclosed in the aforementioned U.S. Pat. No. 4,560,253, the full length of the lens system is relatively small and the zoom lens is compact, but particularly at the wide angle end, the full length is great and an oblique ray incident on the fore lens passes a location farther than the optical axis, and this has led to the disadvantage that the diameter of the fore lens becomes large. Accordingly, this leads to the large size and increased cost of the entire first lens unit. Particularly the large size of the first lens unit including an aspherical lens leads to a further increase in cost, and this is not preferable.
Further, the zoom lenses heretofore proposed as described above are set to such a refractive power arrangement that the full length becomes greatest at the wide angle end and therefore, the amount of movement of each unit becomes great, and this has led to the problem that when the zoom lens is made into a compact construction having a small full length, the designing of the lens barrel becomes difficult and complicated and this leads to an increase in cost.